1. Field of the Invention
The invention relates generally to air pollution control processes aimed at controlling acid gases that are emitted from industrial, utility, incineration, or metallurgical process. Specifically the invention concerns the mitigation of Sulfur Dioxide (SO2), Hydrochloric acid (HCl), and Sulfur Trioxide (SO3) using a high reactivity calcium hydroxide (hydrated lime) in a circulating dry scrubber (CDS).
2. Description of the Related Art
Many efforts have been made to develop materials for improved capability of cleaning or “scrubbing” flue gas or combustion exhaust. Most of the interest in such scrubbing of flue gas is to eliminate particular compositions, specifically acid gases, that contribute to particularly detrimental known environmental effects, such as acid rain.
Flue gases are generally very complex chemical mixtures which comprise a number of different compositions in different percentages depending on the material being combusted, the type of combustion being performed, impurities present in the combustion process, and specifics of the flue design. However, the release of certain chemicals into the atmosphere which commonly appear in flue gases is undesirable, and therefore their release is generally regulated by governments and controlled by those who perform the combustion.
Some of the chemicals that are subject to regulation are certain acid gases. A large number of acid gases are desired to be, and are, under controlled emission standards in the United States and other countries. This includes compounds such as, but not limited to, hydrogen chloride (HCl), sulfur dioxide (SO2) and sulfur trioxide (SO3). Sulfur trioxide can evidence itself as condensable particulate in the form of sulfuric acid (H2SO4). Condensable particulate can also be a regulated emission.
Flue gas exhaust mitigation is generally performed by devices called “scrubbers”. Scrubbers introduce chemical compounds into the flue gas. The compounds then react with the undesirable compounds which are intended to be removed. Through these reactions, the undesirable compounds are either captured and disposed of, or turned into a less harmful compound prior to their exhaust, or both. In addition to controlling the emissions for environmental reasons, it is desirable for many combustion plant operators to remove acid gases from the plant's flue gas to prevent the acid gases from forming powerful corroding compounds which can damage flues and other equipment.
These acid gases can arise from a number of different combustion materials, but are fairly common in fossil fuel combustion (such as oil or coal) due to sulfur being present as a common contaminant in the raw fuel. Most fossil fuels contain some quantity of sulfur. During combustion, sulfur in the fossil fuel can oxidize to form sulfur oxides. A majority of these oxides forms sulfur dioxide (SO2), but a small amount of sulfur trioxide (SO3) can also be formed. Selective Catalyst Reduction (SCR) equipment, commonly installed for the removal of nitrogen oxides (NOx), will also oxidize a portion of the SO2 in a flue gas to SO3.
SO2 is a gas that contributes to acid rain and regional haze. Since the 1970's, clean air regulations have been designed to reduce emissions of SO2 from industrial processes at great benefit to the environment and human health. For large emitters, the use of wet and dry scrubbing has led to the reduction of SO2. Smaller emitters, however, seek out less costly capital investment to control SO2 emissions in order to remain operating and produce electricity or steam. Similarly, halides in fossil fuels (such as chlorine and fluorine) are combusted and form their corresponding acid in the flue gas emissions. The halogenated acids also contribute to corrosion of internal equipment or, uncaptured, pollute the air via stack emissions.
However, mitigation of the above undesirable compounds can be very difficult. Because of the required throughput of a power generation facility, flue gases often move through the flue very fast and thus are present in the area of scrubbers for only a short period of time. Further, many scrubbing materials often present their own problems. Specifically, having too much of the scrubbing material could cause problems with the plant's operation from the scrubber material clogging other components or building up on moving parts.
Flue gas treatment has become a focus of electric utilities and industrial operations due to increasingly tighter air quality standards. As companies seek to comply with air quality regulations, the need arises for effective flue gas treatment options. Alkali species based on alkali or alkaline earth metals are common sorbents used to neutralize the acid components of the flue gas. The most common of these alkalis are sodium, calcium, or magnesium-based. A common method of introduction of the sorbents into the gas stream is to use dry sorbent injections. The sorbents are prepared as a fine or coarse powder and transported and stored at the use site. Dry sorbent injection systems pneumatically convey powdered sorbents to form a fine powder dispersion in the duct. The dry sorbent neutralizes SO3/H2SO4, and protects equipment from corrosion while eliminating acid gas emissions. Common sorbents used are sodium (trona or sodium bicarbonate) or calcium (hydrated lime or Ca(OH)2) based.
One commonly used material for the scrubbing of acid gases is hydrated lime. It has been established that hydrated lime can provide a desirable reaction to act as a mitigation agent. Hydrated lime reacts with SO3 to form calcium sulfate in accordance with the following equation:SO3(g)+Ca(OH)2(s)→CaSO4(s)+H2O(g)
Hydrated lime systems have been proven successful in many full scale operations. These systems operate continuously to provide utility companies with a dependable, cost-effective means of acid gas control.
These hydrated lime compositions specifically focus on high surface area based on the theories of Stephen Brunauer, Paul Hugh Emmett, and Edward Teller (commonly called the BET theory and discussed in S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309, the entire disclosure of which is herein incorporated by reference). This methodology particularly focuses on the available surface area of a solid for absorbing gases—recognizing that a surface, in such circumstances, can be increased by the presence of pores and related structures. The most effective hydrated lime sorbents for dry sorbent injection have high (greater than 20 m2/g) BET surface area.
Two examples of such compositions with increased BET surface areas are described in U.S. Pat. Nos. 5,492,685 and 7,744,678, the entire disclosures of which are herein incorporated by reference. Because of this, commercially available products are currently focused on obtaining lime hydrate with particularly high BET surface areas. It is generally believed that the BET surface area needs to be above 20 m2/g to be effective and, in many recent hydrated lime compositions, the BET surface area is above 30 m2/g in an attempt to continue to improve efficiency. These sorbents offer good conveying characteristics and good dispersion in the flue gas, which is necessary for high removal rates. Use of a higher quality, high reactivity source of hydrated lime allows for better stoichiometric ratios than previous attempts that utilized lower quality hydrated lime originally targeted for other industries such as wastewater treatment, construction, asphalt, and the like.
The reaction of hydrated lime with acid gas (such as SO3) is generally assumed to follow the diffusion mechanism. The acid gas removal is the diffusion of SO3 from the bulk gas to the sorbent particles. Thus, high surface area does not itself warrant a prediction in improved removals of acid gases. Specifically, high pore volume of large pores is generally believed to be required to minimize the pore plugging effect and therefore BET surface area has been determined to be a reasonable proxy for effectiveness of lime hydrates in removal of acid gases. Conventional wisdom also indicates that smaller particles act as better sorbents.
Lime hydrate meeting the above described characteristics, properties, and reactivity has generally been manufactured according to a commonly known and utilized process. First, a lime feed of primarily calcium oxide (commonly known as quicklime) is continuously grinded using a pulverizing mill until a certain percentage of all the ground particles meet a desired size (e.g., 95% or smaller than 100 mesh). In other words, all of the lime feed is ground together (lime and impurities), without any removal of particles during the grinding, until the batch of lime feed (both the lime and impurities) meets the desired particle size requirements. This continuous grinding is not surprising as the conventional wisdom is that small particles are better and, thus, the more the calcium oxide is grinded, the better.
Second, the quicklime meeting the desired size requirements is then fed into a hydrator, where the calcium oxide reacts with water (also known as slaking), and then flash dried to form calcium hydroxide in accordance with the following equation:CaO+H2O→Ca(OH)2 
Finally, the resultant calcium hydroxide (also known as hydrated lime) is then milled and classified until it meets a desired level of fineness and BET surface area.